This is probably the sole reason AT&T is going to buy T-Mobile. AT&T is currently the only GSM carrier to have the iPhone contractually, but that doesn't mean it's the only GSM carrier in the USA (yet.) If you've got an unlocked iPhone, you can take it to any GSM carrier in the country and get service for it, albeit without 3G in some cases.
T-Mobile today let the world know that it's currently serving a whopping one million iPhones on its network. That's huge, especially since the iPhone is not sold in T-Mobile stores, nor is it compatible with that carrier's 3G (or 4G) bands. This can mean one or both of two things: 1) AT&T just plain sucks that bad or 2) T-Mobile's plans are much better than those of AT&T's. Either way, this shows that the merger will probably be bad for a lot of people. But we already knew that.
This doesn't mean just iPhone 4s, however; it covers all iPhones that have ever existed. However, it still shows just how many people actually voided their warranties to get better service.[GottaBeMobile]